Time-resolved absorption spectroscopy is used to study the dynamics of protein structural changes subsequent to excitation with short laser pulses. Molecular models for the protein dynamics are used to fit and interpret the measured data. A. The kinetics of ligand binding and conformational changes for sperm whale myoglobin in sugar glasses have been studied at temperatures from 100K to 300 K following the photodissociation of carbon monoxide from the hemes. The results confirm the prediction, based on work of Ansari et al. (1992,1994) that conformational relaxation can be inhibited by increasing the solvent viscosity. B. The dynamics of short-lived states on the folding pathway of cytochrome c have been investigated using time-resolved absorption spectroscopy. The binding of carbon monoxide destabilized the folded state of reduced cytochrome c, permitting folding to be initiated by photodissociation of this ligand. We have observed transient binding of other at least two different side chains to the heme subsequent to photodissociation of CO. C. The kinetics of intracellular polymerization of hemoglobin S have been studied for cells from patients undergoing treatment with hydroxyurea. The delay times increased dramatically in the course of treatment, with the median value increased by a factor of 5 after 12 weeks of treatment and more than a factor of 10 after 24 weeks.